1. Field of the Invention
The present invention relates to a system and method to control a powertrain of a vehicle, and in particular powertrain control during vehicle deceleration.
2. Background of the Invention
A vehicle powertrain typically includes a power source such as an internal combustion engine, coupled to a gearbox such as a transmission with various gear ratios. A powertrain controller is usually provided which controls the powertrain in part based on a driver's requests.
In one system, desired powertrain output is determined based on depression of a pedal by a vehicle operator. In such a system, a strategy is needed for the situation when a driver releases the pedal, known to those skilled in the art as a tip-out. During a tip-out, the driver is indicating a desire for reduced engine output.
The inventors herein have recognized a disadvantage if conventional methods of powertrain output control are used during tip-out situations. In particular, if the accelerator pedal is released and subsequently engaged, poor drivability may be experienced due to transmission gears lash. For example, when the engine transitions from exerting a positive torque to exerting a negative torque (or being driven), the gears in the transmission separate at the zero torque transition point. Then, after passing through the zero torque point, the gears again make contact to transfer torque. This series of events produces an impact, or clunk, resulting in poor drivability and customer dissatisfaction. In other words, the engine first exerts a positive torque through the torque converter onto the transmission input gears to drive the vehicle. Then, in response to the driver tip-out, the engine is driven by the torque from the transmission through the torque converter. The transition between these two modes is the point where the engine is producing exactly zero engine brake torque. Then, at this transition point, the gears in the transmission separate because of inevitable transmission gear lash. When the gears again make contact, they do so dynamically, resulting in an undesirable impact.
This disadvantage is exacerbated when the operator returns the accelerator pedal to a depressed position, indicating a desire for increased powertrain output. In this situation, the zero torque transition point must again be traversed. However, in this situation, the engine is producing a larger amount of torque than during deceleration because the driver is requesting acceleration. Thus, another, more severe, impact is experienced due to the transmission lash during the zero torque transition.
The inventors herein have recognized another disadvantage if conventional methods of powertrain output control are used during tip-out situations. In particular, different vehicle trajectories are obtained depending on road grade. Stated another way, if a desired powertrain output torque is requested, different vehicle deceleration rates are obtained depending on road grade. At steep down grades, the vehicle may accelerate even if negative powertrain torque is requested due to vehicle mass. If the conventional control strategy is changed to minimize this acceleration on down grades, then excessive deceleration is obtained on flat road grades. Thus, drive performance is sacrificed under one driving condition or another.